Planetary Formation
According to the condensation theory of planet formation, planets form out of a spinning disk of gas that surrounds a newborn star known as a protoplanetary disk. The disk and star both originate in a rotating, collapsing cloud of material, and this process of collapse produces different abundances of materials in the disk at different distances from the star. In the higher temperature regions, comparable to the region around the planet Mercury in our solar system, the only kinds of material that can condense from the gas to the solid state (in this case, microscopic dust grains) are metals. Farther out, about where Venus, Earth, and Mars are now, the gas temperatures are lower. At these distances and temperatures, rocky materials such as silicates can also begin to form dust grains. Even farther out, the temperature gets low enough for water ice to form, and even farther from the star, ices of other compounds such as ammonia and methane can condense But how do young planetary systems go from making dust grains to making planets?
“The answer to that question, “explains astronomer David Jewitt, “is a process called binary accretion, where collisions between pairs of objects let larger and larger structures get put together. Collisions between grains, which are small and sticky, quickly lead to the construction of pebbles. Collisions between pebbles lead to rocks. Collisions between rocks lead to boulders. Collisions between boulders lead to planetesimals, rocky bodies the size of asteroids (bodies that orbit between Mars and Jupiter and range in size up to about 1,000 kilometers). If this accretion process happens far enough from the star, significant quantities of ices will be included in the planetesimals. This is the likely origin of comets. Eventually, the objects are large enough for gravity to begin compressing and heating the planetesimal interiors. As planetesimals grow even larger, gravity pulls them into a spherical shape, and the heaviest elements sink to the center of the body. Iron and nickel will, in this way, form the dense metallic cores of the young planets Eventually, full-sized terrestrial planets, the rocky cores of gas giants(such as Jupiter and Saturn), and moons are formed through the accretion process.
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